Need to turn CEDB into a trapezium shape, the area of a trapiezium is (a+b)/2)*h
= ((2-2cos(theta)/sin(theta))+(2/sin(theta))/2*(2)
The 2*2 cancels out, and hence it becomes ((2-2cos(theta)/sin(theta))+(2/sin(theta))
Which simplifies to (4-2cos(theta)/sin(theta))
Using the quotient rule to differentiate
Le u = 4-2cos(theta) let v = sin(theta)
Hence the derivative is ((-4cos(theta) + 2cos^2(theta)+2sin^2(theta))/(sin^2(theta))
Derivative = 0 for minimum
Hence ((-4cos(theta) + 2cos^2(theta)+2sin^2(theta))/(sin^2(theta)) = 0
Hence ((-4cos(theta) + 2cos^2(theta)+2sin^2(theta)) = 0
Hence cos(theta)(-4+2cos(theta)) + 2sin^2(theta) = 0
We know that sin^2(theta) + cos^2(theta) = 1So -4cos(theta) + 2(cos^2(theta)+sin^2(theta)) = 0
So -4cos(theta) + 2 = 0
So cos(theta) = ½
Base = pi/3
Cos is positive in quadrant 1 and 4
And the domain for theta is from 0 to pi/2
Hence theta = pi/3
When be sub pi/3 into (4-2cos(theta)/sin(theta))
We get the minimum area as 2root3
This question took me 10 minutes to do when I was doing it on the exam lol
Hope this helps
